1. Field of the Invention
Embodiments presented in this disclosure generally relate to identifying a location of an RF tag and, more specifically, to determining a range of the tag using continuous wave radar.
2. Description of the Related Art
Typical radio frequency identification (RFID) tags or RF tags include a microprocessor functionally connected to an antenna. The microprocessor stores and processes relevant data that may include unique data for identifying a specific item associated with the RF tag. The microprocessor also modulates a radio frequency (RF) signal that is transmitted or backscattered via the antenna. An external tag reader captures the data signal transmitted by the RF tag.
RF tags can be classified as “active” or backscatter devices. Active tags use an internal power source to actively transmit a modulated signal to the tag reader. Backscatter tags, in contrast, do not actively transmit modulated signals to the tag reader but modulate the electromagnetic waves scattered from the tag's antenna. The signal that is scattered originates from the tag reader. Backscatter tags can be classified as “passive” (without any onboard power source) or “semi-passive” (includes an internal power source but still communicates with the tag reader via modulated backscatter and does not actively transmit a signal to the tag reader). Passive RF tags are powered by the incident signal transmitted by the tag reader. Passive and semi-passive tags are typically less expensive than active tags but have shorter functional ranges than active RF tags. Backscatter tags typically operate in the ultra-high frequency (UHF) or microwave frequency bands.
In some cases, the external tag reader may be configured to determine the distance between the reader and the RF tag. Conventional ranging techniques, such as received signal strength (RSS) for either active or passive RF tags, work over relatively large areas but suffer from poor accuracy. Time of flight or frequency modulated continuous wave radar may also be used to determine the separation distance but these techniques work poorly for short distance applications because of the difficulties of measuring the small round trip time or frequency delay. Instead RF phase-based ranging techniques are preferred when high accuracy is required.